1. Field
Disclosed herein is a method for producing an electricity sensing device such as, for example, an electricity meter or energy meter.
2. Description of Related Art
A variety of electronic electricity meters (or electric meters in US parlance) is known for sensing electricity or energy which are now increasingly taking the place of the mechanical Ferraris meters in industry and domestic applications and which implement electricity sensing by mechanical and electrical assemblies in diverse configurations. In addition to electricity sensing by means of measuring shunts, Rogowski coils or Hall elements, current transformers based on soft magnetic ring cores, especially ring band cores, are popular as magnetic modules in electricity meters. A magnetic module (current transformer) DC decouples the power to furnish a precise measurand in the form of a signal voltage across a burden resistor. The requirements as to the accuracy of amplitude and phasing and linearity are specified by IEC 62053, -21, -23, formerly 1036 in Europe, and ANSI C12.xx in the USA as cited, for example, in the company prospectus “VAC current transformers for electronic energy meters” of the German firm Vacuumschmelze, published October 1998. Current transformers for electronic energy meters are also cited generally in the company prospectus “Current transformers for electronic energy meters” of the firm Vacuumschmelze, published 2002. Such electricity meters employing current transformers (also termed Watthour meters) serve as officially approved means of measuring the electrical current representing the energy consumption as billed by power utilities.
A busbar structure forming so-called primary conductors together with a compatible ring core current transformer for sensing the consumption amperage are typically used. Plug-in electric meters popular in the USA and other countries feature standardized rear rectangular terminals for plugging into mating spring contacts when mounting the meter. These contacts, with a cross-section of approximately a×2.5 mm serve to input and output the consumption amperage, which on 110 V systems amounts to a maximum of approximately 200-480 Arms. Factor “a” represents the thickness of the cross-section and is set at a=19 mm for a maximum current of Imax=320 A. It is usually the case that the currents of the three phases of the AC power grid are conducted into the electricity meter through an electricity sensing system and back out of the electricity meter.
The current transformer may be configured so that a busbar dimensioned 19×2.5 mm, for example, can be inserted through a hole in the interior of the current transformer. The portion of the busbar for mounting the current transformer may also have a round cross-section so that the hole in the current transformer can be dimensioned smaller, making it possible to use a smaller and less costly ring band core. Even though the time required to produce the core and make the windings is the same, the processing steps involved in heat treatment and coating become all the more favorable the smaller the diameter of the core. Producing a busbar suitable for this purpose is done by providing a U-shaped assembly of conductors with diverse portions. A central connecting portion having a round cross-section serves as the element of the current transformer for passing through the corresponding opening in the core. Two terminal portions having a rectangular cross-section serve to connect the current conductor in the form of plug-in connectors known as such, as already explained above.
When fitting the current transformer to a one-piece primary conductor it is a mandatory requirement to mount the inductive transformer on the primary conductor together with the terminal contacts thereof. This automatically results in the minimum inner diameter of the magnetic transformer being dictated by the size of the plug-in contact for a primary conductor made in one piece.
Although it is possible to adapt the inner diameter of the inductive current transformer to the minimum possible by the electromagnetic design, when the primary conductor comprises a plurality of separate parts, this adds to the complications in assembling the primary busbar. The conductor assembly in this arrangement is made up of three metal parts each differing in cross-section from the other, the two ends of the current conductor needing to be secured to the flats of the rectangular connecting leads. The methods as usual for jointing busbars made up of three separate parts, for example, are brazing and welding. Both of these methods make it necessary to protect the current transformer from the heat generated in jointing, this in turn necessitating complicated designs with cooling clamps between the jointing location and current transformer.
Another drawback of these methods is the highly restricted possibility of checking proper jointing. Indeed, checking the joint for absolute assurance is only possible by destructive testing. In addition to this, there is a risk of electrochemical corrosion of the joint due to the difference in the normal potentials of the alloys used, as is especially the case with brazed connections depending on the combination of solder and conductor material employed. This risk is especially to be avoided where outdoor energy meters are involved, as is usual in the NAFTA area, where the influence of moisture, possibly in combination with industrial toxic emissions such as e.g. NOx or SOx compounds is to be reckoned with.
To get round these difficulties involved in thermal jointing methods it was proposed, for example, in German patent DE 10 2004 058 452, to implement jointing by cold-press welding. Although this method avoids the heating-up of the joint, the resulting joints of the separate components of the primary conductor have other disadvantages. For instance, only a fraction of the terminal pad comprises cold-press welded material. The majority of the connecting surface is merely positively connected, resulting in a micron air gap remaining between the partners of the joint. This air gap reduces the current-carrying capacity of the joint, resulting in the risk of the joint becoming over-heated when the conductor is loaded to a maximum.
The connections of such a conductor assembly of three elements having cross-sections, each differing from the other at the points of connection, are intended to reliably achieve a long life of, for example, 10-15 years, thus demanding the processes in fabricating the conductor assembly to be safe and sound. For good electrical conductivity the corresponding busbars or conductor assemblies are mainly structured in a copper material, causing problems, however, both with brazing and welding, particularly due to the heat in making the joints, because copper is a good thermal conductor, so that the heat is transmitted by the current conductor to the current transformer, risking damage thereto.